With depletion of global liquid petroleum reserves, natural gas, containing primarily methane, is expected to be one of the main resources for the production of liquid fuels. However, direct dehydrogenation of light alkanes like methane and ethane to more valuable petrochemical products, e.g. olefins, aromatics (hereafter denoted olefin/aromatics) remains challenging.
For ethane to olefin/aromatics production, commercial processes include steam cracking and catalytic dehydrogenation, and recently there has also been renewed interest in oxidative dehydrogenation.
Oxidative dehydrogenation offers direct conversion from alkanes into valuable chemicals. By adding oxygen discretely through either porous or dense oxygen permeable membranes, the alkane to oxygen ratio can be kept high promoting high C2+ selectivity.
A somewhat less investigated route for alkane conversion to fuels is through non-oxidative reactions. Here, using methane as an example, a coupling/dimerization/pyrolysis (hereafter denoted coupling) reaction takes place on the methane side of a reactor with hydrogen permeating through a membrane in the form of protons onto the oxygen side, where it reacts with oxygen to form water.
Oxygen is not present in the methane coupling compartment, avoiding the oxidation of methane. A high C2+ selectivity may thus be expected. This is a highly efficient way to make olefins/aromatics from alkanes compared to existing technologies.
It has been shown theoretically that removal of hydrogen during coupling promotes homogeneous reaction pathways and shifts the equilibrium towards the product side. A hydrogen selective membrane in the process stream should therefore increase the yield considerably. The removal of hydrogen can be achieved using hydrogen permeable membranes.
Several such membranes exist. Catalytic dehydrogenation of ethane in a hydrogen membrane reactor has been investigated using a microporous silica membrane and a 5.0 wt. % Cr2O3/γ—Al2O3 catalyst prepared by incipient wetness impregnation of a γ—Al2O3 support.
A Pd—Ag composite membrane supported on porous stainless steel prepared by electroless plating has been used in a catalytic membrane reactor utilizing a Ru—Mo/HZSM-5 catalyst.
Using the ceramic mixed proton-electron conductor SrCe0.95Yb0.05O3−δ a membrane configuration and also a co-generative fuel cell has been developed towards methane coupling.
There are problems with all these solutions however. Microporous membranes suffer from being fragile and difficult to make. Their hydrogen selectivity is also poor.
Pd—Ag membranes are inherently very expensive and whilst complex membranes have been formed in an attempt to minimise expensive metal content, there remains a desire to have a much simpler membrane. The catalytic activity of these metals towards formation of coke is also a considerable problem if these materials are used in a catalytic membrane reactor.
Ceramic oxides offer a more attractive option therefore. However, even initiatives using ceramic proton conducting materials have serious limitations. The prior art ceramic oxides are based on Ba- and Sr-based perovskites. These compounds are basic and react with CO2 and H2S/SO2/SO3 at moderate temperature and H2O at low temperatures to form alkaline earth carbonates, sulphates and hydroxides, respectively. Consequently, a decrease in conductivity is observed.
These reactions are prohibitive if using any carbon-containing feed gas as the impurities in the gas react with the membrane. Moreover, the reaction with carbon dioxide precludes the use of air in a reactor meaning expensive inert gases have to be used. Moreover, the electrical and mechanical properties of these materials become poor due to the formation of carbonates and hydroxides.
There remains therefore a need to develop new membrane materials which avoid the problems of the prior art. The inventors have found that a membrane based on mixed metal tungstates offers an ideal solution to this problem. These materials are stable in the presence of carbon dioxide and acidic gases in general making them usable in the presence of air. This also means the membranes can be used in the presence of hydrocarbon feed gases.
Moreover, the inventors have realised that the tungstates form an ideal membrane as they offer just the right hydrogen selectivity for an alkane to alkene, or more generally to olefin/aromatic dehydrogenation process. If too much hydrogen is allowed through the membrane, that simply encourages the equilibrium of the reaction to move too far to the right and hence to the formation of carbon itself. There are in fact membranes with better hydrogen selectivity but the use of such membranes in this reaction is actually detrimental. The membranes of the present invention ensure that the amount of hydrogen which passes through the membrane is sufficient to allow alkene (olefin/aromatic) formation but not coke formation.
It is an important feature of the invention that the inventors have appreciated that the proton membrane of the invention should not be too good a proton conductor as that is actually a problem rather than an advantage as it encourages coke formation and not alkene formation.
The mixed metal oxide used in the membrane is not itself new. In Solid State Ionics, 143 (2001), 117-123, the authors investigate the proton conducting properties of lanthanum tungstates. The present inventors have realised that these proton conducting materials, as opposed to the numerous other proton conducting materials known, offer the most attractive properties for use in dehydrogenation reactions, in particular of alkanes to alkenes (olefins/aromatics).
Thus, viewed from one aspect the invention provides a reactor comprising a first zone comprising a dehydrogenation catalyst and a second zone separated from said first zone by a proton conducting membrane comprising a mixed metal oxide of formula (I)LnaWbO12−y  (I)wherein Ln is Y or an element numbered 57 to 71;the molar ratio of a:b is 4.8 to 6, preferably 5.3 to 6; andy is a number such that formula (I) is uncharged, e.g. y is 0≦y≦1.8.
Viewed from another aspect the invention provides a reactor comprising a first zone comprising a dehydrogenation catalyst and a second zone separated from said first zone by a proton conducting membrane comprising at least one mixed metal oxide of formula (II)LnaWb−cMocO12−y  (II)wherein Ln is Y or an element numbered 57 to 71;the molar ratio of a:b is 4.8 to 6, preferably 5.3 to 6; c is 0 to (0.5×b); andy is a number such that formula (II) is uncharged, e.g. y is 0≦y≦1.8.
Viewed from another aspect the invention provides a process for the dehydrogenation of substance, e.g. an alkane, comprising introducing said substance into the first zone of a reactor as hereinbefore defined to thereby dehydrogenate said sub stance;
allowing hydrogen formed during said dehydrogenation to pass through said proton conducting membrane into said second zone;
introducing a purge gas into said second zone, preferably to react with the hydrogen; or
applying reduced pressure in said second zone to thus remove hydrogen from said second zone.
Viewed from another aspect the invention provides a proton conducting membrane comprising a dehydrogenation catalyst and a mixed metal oxide of formula (I)LnaWbO12−y  (I)wherein Ln is Y or an element numbered 57 to 71;the molar ratio of a:b is 4.8 to 6, preferably 5.3 to 6; andy is a number such that formula (I) is uncharged, e.g. y is 0≦y≦1.8.
Viewed from another aspect the invention provides a proton conducting membrane comprising a dehydrogenation catalyst and a mixed metal oxide of formula (II)LnaWb−cMocO12−y  (II)wherein Ln is Y or an element numbered 57 to 71;the molar ratio of a:b is 4.8 to 6, preferably 5.3 to 6; c is 0 to (0.5×b); andy is a number such that formula (II) is uncharged, e.g. y is 0≦y≦1.8.
Viewed from another aspect the invention provides the use of a proton conducting membrane as hereinbefore defined in a dehydrogenation process.